A conventional photovoltaic cell structure with a p-type base has a negative electrode that is typically on the front side, i.e., sun side or illuminated side, of the cell and a positive electrode on the back side. Radiation of an appropriate wavelength falling on a p-n junction of a photovoltaic cell serves as a source of external energy to generate hole-electron pairs in that device. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to the flow of an electric current that is capable of delivering power to an external circuit. Most photovoltaic cells are in the form of a silicon wafer that has been metalized, i.e., provided with metal contacts that are electrically conductive.
Most electric power-generating solar cells currently used are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing a metal paste and subsequent firing.
Typically, an anti-reflection coating (ARC) is formed on the front side of a solar cell. Exemplary anti-reflection coating materials useful in the methods and devices described herein include, but are not limited to: silicon nitride, silicon oxide, titanium oxide, SiNx:H, hydrogenated amorphous silicon nitride, and silicon oxide/titanium oxide film. The coating can be formed by plasma enhanced chemical vapor deposition (PECVD), chemical vapor deposition (CVD), thermal CVD or other known techniques. In an embodiment in which the coating is silicon nitride, the silicon nitride film can be formed by low pressure CVD, PECVD, thermal CVD, or physical vapor deposition (PVD). In an embodiment in which the insulating film is silicon oxide, the silicon oxide film can be formed by thermal oxidation, thermal CVD, plasma CVD, or PVD.
For acceptable performance the front side electrode must make good electrical contact with the silicon cell surface adjacent to the anti-reflection coating. Two approaches have been used to accomplish this contact. One approach has been to deposit a front electrode-forming electrically conducting paste, e.g., a silver paste, onto the ARC. During firing the silver paste sinters and penetrates through the ARC and the front electrode so made is thereby able to make electrical contact with the silicon cell. Alternatively, the ARC has been etched in the portions where the electrode is to make contact with the silicon cell.
The silver paste is then deposited into those portions and fired to form the electrode. Etching has been accomplished by using lasers which is expensive and by using etching compositions such as Isishape® Solaretch® products available from Merck KGaA, Darmstadt, Germany.
There is a need for a non-corrosive etching composition that can remove selected portions of an anti-reflection coating and particularly for such a composition that is both thermal and UV curable.